An ion trap is a widely used mass spectrometer, accumulates ions, and thereafter ejects the ions mass-selectively. A configuration of the ion trap and a measurement method are described in Patent Documents 2 to 5. In the ion trap, ions introduced from an ion source are released while a mass spectrometry is being performed, which leads to a loss. Thus, there is a problem of low duty cycle. If the ions introduced from the ion source while the mass spectrometry is being performed with the ion trap can be used for the mass spectrometry, the sensitivity of the ion trap can be enhanced. Patent Document 1 describes a method by which the duty cycle is enhanced in the following manner. Specifically, while the mass spectrometry is performed with an ion trap, ions introduced from an ion source are accumulated in a two dimensional multipole electric field formed with multipole rods. Then, the ions are introduced into the ion trap in a step of accumulating the ions in the ion trap. In addition, Patent Document 2 describes a method by which the duty cycle is enhanced by mass-selectively ejecting ions at the same time while accumulating the ions in an ion trap.